Dario,
great detective work - and more remarkable than I would have guessed (although I've been a fan for years of the shifted grids). What Stefano said so clearly is related to the original idea of Alfonso Baldereschi (PRB 7, 5121 (1973)) and generalized by Chadi and Cohen (PRB 8, 5747 (1973) - interestingly written in Paris VI Jussieu) - i.e. how to choose a single, special k-point (Baldereschi) or meshes of them (Chadi and Cohen) in order to get the most accurate average. Basically, a function f(k) that has the symmetry f the crystal, fourier expanded in plane waves exp (i k.R) (R lattice vectors), can be rewritten as a linear combination of symmetrized stars of plane waves (you bunch together all the plane waves that have the same modulus for R, paying some attention to degenerate shells that have different symmetry). By choosing k's such that the largest number of those symmetrized stars (sum exp (i k.R), sum over the R of a star) are zero, you have a recipe that gives accurate integrals Stefano de Gironcoli wrote: > ... this annoying fact is the origin of the infamous > nosym=.true. options that much confusion generates in most users until > they realize what its meaning and purpose really is ... there are many > threads on that in the archive.... A god-blessing ! Try it - instead of using a 2x2x2 shifted mesh (4 k-points, if no symmetry is present beside time reversal) use only 1 point, and stop the code from symmetrizing it using nosym=true. The results will be very close to the full mesh, at 1/4 of the cost. nicola --------------------------------------------------------------------- Prof Nicola Marzari Department of Materials Science and Engineering 13-5066 MIT 77 Massachusetts Avenue Cambridge MA 02139-4307 USA tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu